An electron microscope study of resin production and secretion by the glands of seedlings of Betula pendula Roth

Transmission and scanning electron microscopy were used to study the ultrastructure and secretory processes of resin glands on shoot stems of Betula pendula seedlings during seasonal growth. The multicellular peltate glands possess a cortex of columnar cells surrounding a parenchymal medulla differing from the stem parenchyma below. Myelin-like deposits comprising concentric layers of membranes and osmiophilic substances accumulate mainly in the cortical cells, while only the medullar cells have chloroplasts. Both of these deposits appear to be synthesized, initially, in the endoplasmic reticulum. The myelin-like material is believed to consist of steroidal triterpenoids in cytoplasmic membranes, and the osmiophilic deposits to represent phenolics. Studies of glands of different ages suggest that the electron-transparent resin ultimately exported is formed by combining the two initial products. In the cortical cells the secretion first accumulates in vesicles that fuse with larger ones and the periplasmatic space. From the latter the secretion diffuses through the cell wall and the resin is finally deposited in the subcuticular space. Secretion vesicles, but no periplasmatic deposits were seen in the medullar cells. At the end of seasonal growth the cortical cells are markedly vacuolate and appear to have lost their organelles. Some of the cells accumulate an electron-opaque material not seen earlier. The medullar cells of the glands are suberized before winter dormancy, but usually later than in the surrounding stem bark.